New Insights into Development of Transglutaminase 2 Inhibitors as Pharmaceutical Lead Compounds.

Soo-Youl Kim

doi:10.3390/medsci6040087

Abstract

Transglutaminase 2 (EC 2.3.2.13; TG2 or TGase 2) plays important roles in the pathogenesis of many diseases, including cancers, neurodegeneration, and inflammatory disorders. Under normal conditions, however, mice lacking TGase 2 exhibit no obvious abnormal phenotype. TGase 2 expression is induced by chemical, physical, and viral stresses through tissue-protective signaling pathways. After stress dissipates, expression is normalized by feedback mechanisms. Dysregulation of TGase 2 expression under pathologic conditions, however, can potentiate pathogenesis and aggravate disease severity. Consistent with this, TGase 2 knockout mice exhibit reversal of disease phenotypes in neurodegenerative and chronic inflammatory disease models. Accordingly, TGase 2 is considered to be a potential therapeutic target. Based on structure–activity relationship assays performed over the past few decades, TGase 2 inhibitors have been developed that target the enzyme’s active site, but clinically applicable inhibitors are not yet available. The recently described the small molecule GK921, which lacks a group that can react with the active site of TGase 2, and efficiently inhibits the enzyme’s activity. Mechanistic studies revealed that GK921 binds at an allosteric binding site in the N-terminus of TGase 2 (amino acids (a.a.) 81–116), triggering a conformational change that inactivates the enzyme. Because the binding site of GK921 overlaps with the p53-binding site of TGase 2, the drug induces apoptosis in renal cell carcinoma by stabilizing p53. In this review, we discuss the possibility of developing TGase 2 inhibitors that target the allosteric binding site of TGase 2.

Highlights

We discuss the possibility of developing TGase 2 inhibitors that target the allosteric binding site of Keywords: transglutaminase 2; inhibitor; conformation; dimer; polymerization; cross-linking
More reactions and biological functions of TGase 2 are discussed in the reviews [1,15] including nuclear factor κB (NF-κB) activation through NF-κB inhibitor α (I-κBα) inhibition [16], hypoxia-inducible factor 1α (HIF-1α) activation through von Hippel–Lindau tumor suppressor (VHL) inhibition [17] and suppression of apoptosis in cancer through p53 inhibition [15,18]
P53 binding itself is not sufficient to form a polymer of p53, which requires the high concentration of calcium present in the autophagosome [60]. These observations suggest that TGase 2 acts as a chaperone of p53 with cross-linking activity. This p53- TGase 2–p62 interaction of triple complex has an advantage of rapid autophagy to compare to mouse double-minute 2 homolog (MDM2) mediated p53 ubiquitination, which is directly destined to microtubule-associated protein light chain 3 (LC3) on the membrane of autophagosome through p62 binding in renal cell cancer (RCC) [30]

Summary

Introduction

More reactions and biological functions of TGase 2 are discussed in the reviews [1,15] including nuclear factor κB (NF-κB) activation through NF-κB inhibitor α (I-κBα) inhibition [16], hypoxia-inducible factor 1α (HIF-1α) activation through von Hippel–Lindau tumor suppressor (VHL) inhibition [17] and suppression of apoptosis in cancer through p53 inhibition [15,18]. The transglutaminase 2 (TGase 2, E.C. 2.1.2.13) inhibitor GK921, 3-(phenylethynyl)-2-(2(pyridin-2-yl)ethoxy)pyrido[2,3-b]pyrazine, eliminates renal cell cancer (RCC) in a xenograft model by inhibiting TGase 2 [25] It does not contain a ‘warhead’ that attacks the active site of the enzyme [19]. The TGase 2 quadruple point mutant (Q95A, Q96A, Q103A, R116A) cannot bind GK921 [24]

Is TGase 2 active as a Monomer?

Does TGase 2 Change Conformation by Activation?

Dynamics of TGase 2 Conformation and Activity

Proposal

TGase 2 Dimer Formation and Activity

Multiple Complexes of TGase 2 in the ECM

Triple Complex of TGase 2 in Cancer

Discovery of Allosteric Site of TGase 2

Findings

Conclusions and Future Perspectives